Role of BPO4 flux in BaBPO5 crystal growth characterized through high-temperature Raman spectroscopy

In this study, we explored the growth of incongruently melting crystals in a starting solution with flux to better understand the role of flux in crystal growth mechanisms. We recorded the spectrum of BaBPO₅ crystals across a 200–1400 cm⁻¹ range for detailed Raman spectroscopic analysis. The observed Raman bands corresponded to the vibrations of PO₄ and BO₄ units. These units link with B–O–P bonds via common corners to form PBO₇ groups that serve as basic structural units of the crystal. Notably, these PBO₇ structures remain stable at high temperatures up to the crystal melting point, at which the isolated PO₄ groups predominantly exist in the melt, as revealed by high-temperature Raman spectroscopy. Further investigations into the Raman spectra of the starting solution enriched with BPO₄ self-flux indicate that PBO₇ groups are crucial for crystal growth. The addition of PBO₄ flux aids in the formation of these PBO₇ units. This research paves the way for expanding our understanding to other systems, which will help elucidate the mechanisms behind crystal growth.